The so-called “dip-coating” technique is known, which method is both simple and effective for depositing a coating on the surface of an object. According to this technique, after any preparation of the surface, the object to be coated is immersed in a bath comprising the product to be deposited on said object. The object is next extracted from the bath with the excess liquid being removed, and the coating is made solid, for example by wiping, solidification, polymerization, etc.
One of the most widespread applications of this technique is the coating of steel parts such as strips or wires using a metal such as zinc that will next be used for protection against corrosion.
After passing in the liquid-metal bath, the coated part undergoes the wiping operation. This operation is one of the most important operations in the dip-coating method, since it allows to control the final thickness of the coating. On the one hand, the wiping must be homogeneous over the entire surface of the product, i.e., the width for a strip and the circumference for a wire, and over the entire length of the product to be coated. At the same time, this operation must strictly limit the deposition to the target value, which is typically expressed either in terms of deposited thickness—typically from 3 to 50 micrometers—, or by weight of the deposited layer per surface unit—typically in grams/square meter.
Currently, wiping is generally achieved using gas blades or jets, linear in the case of strips and circular in the case of wires, released from slits and most often oriented perpendicular to the surface to be treated. The gas blades act as “pneumatic scrapers” and have the advantage of operating without mechanical contact and therefore without any risk of scratching the treated object. Such blades are called “gas wipers” or “gas knives”. The compressed gas implemented is either air, or a neutral gas such as nitrogen in the most delicate applications, such as the treatment of steel strips intended for the manufacture of visible parts for motor vehicle bodies.
The final thickness of the coating in particular depends on the speed of motion of the strip, on the distance between the strip and the gas knives, and lastly, on the action exerted by the compressed gas jet on the strip.
Yet, it is known that when the strip passes over the sink roll, it assumes the form of a tile. This plastic deformation must be corrected by means of a second roll, called a decambering roll, which imparts a reverse plastic deformation to the strip. Secondarily, a third roll, called a stabilizing roll, allows to fix the pass line independently of the decambering. However, poor control of the nesting of the rolls causes residual deformation and therefore deteriorated flatness.
Other phenomena can also alter the flatness of the strip. This may involve heterogeneous quality of the base steel, deteriorated rolling conditions or heating conditions, non-homogeneous cooling and soaking temperature during the annealing cycle of the strip, before it enters the liquid-metal bath.
Furthermore, some characteristics of the facility, such as the presence of cooling devices before the top roll, the off-centered nature of certain rolls, the wear of the rolls or that of the bearings of the immersed rolls, etc., cause vibrations of the strip passing in the wipers.
Ultimately, these flatness defects and these vibrations cause variations in the thickness of the coating that affect the quality of the product and entail zinc overconsumption in order to guarantee a minimal coating thickness for the client.
Furthermore, for a given coating thickness, it is necessary to increase the gas wiping pressure when the speed of the strip increases. Yet, it is known that the motion of the strip cannot exceed a critical speed, beyond which splashing occurs: droplets are torn from the wiping wave and are projected on the surface of the bath and on the equipment. This results in significant deterioration of the quality of the product as well as considerable increase in the volume of scum at the surface of the bath.
To solve these problems, builders have proposed to use pneumatic or electromagnetic devices for decambering and stabilizing the strip or still other devices allowing to avoid splashing. It has also been proposed to mount immersed rolls on ceramic bearings or roller bearings.
Document JP 56 153136 A proposes to arrange at least one pair of pneumatic stabilizers or dampers in positions such that the vibrating length decreases between the sink roll and the top roll, which are fixed points for the strip.
Document JP 56 084452 A proposes to use a pneumatic stabilizer in which part of the injected fluid flows along the strip in the direction opposite that coming from the wipers.
Document JP 2005298908 A proposes to avoid splashing by combining a pneumatic cushion with a scraper, where the gas mixes with the liquid to pass under the scraper.
The goal being to stabilize the strip in the wipers, it is necessary for this type of stabilizer to be located in their vicinity, which involves blowing a compressed gas over a coating having a definitive thickness, but not yet solidified, which risks affecting the appearance of the final product. Moreover, these devices do not guarantee the flatness of the strip at the wipers.
Still other devices for hydrodynamic stabilization have been proposed, like in document WO 03/054244 A1. However, this method requires injecting liquid metal into a pipe using a pump. Furthermore, the width of the pipe by which the strip is engaged does not necessarily adapt to the format of the strip, to the coating rate or to the motion speed of the strip.
Furthermore, a certain number of methods are also known for controlling or suppressing vibrations affecting a metal strip in continuous motion based on the implementation of electromagnetic means (see e.g. documents JP 10 298728 A, JP 5 001362 A, JP 9 143652 A, JP 10 87755 A, JP 8 010847 A).
The electromagnetic methods are based on the following principle. Conductors in which a high-frequency current flows are installed on both sides of the steel strip. They induce currents in phase opposition in the strip, Foucault currents. The interaction between the inducing currents and the induced Foucault currents generates a magnetic pressure tending to stabilize the steel strip. Another solution consists in using electromagnets. However, methods of this type involve additional control due to the magnetic attraction force, which tends to make the strip unstable. Moreover, it is known that the high-frequency currents implemented cause a temperature increase in the strip, which is contrary to what is intended in this step of the method.
The teaching of these various techniques does not allow to completely eliminate vibrations or lack of flatness of the strip, which, even if lessened, generally remain at the gas knives. It is therefore in this location that action should be taken, without altering the formation of the coating.